1. Field of the Invention
The present invention relates an alloy-coated gas turbine blade exhibiting a high-temperature durability and especially a high-temperature anticorrosive property, a manufacturing method thereof and a gas turbine including the same gas turbine blade.
2. Related Background Art
A gas turbine for power generation aims at improving a power generation efficiency, and a temperature of a combustion gas is therefore increased. As a result, it is highly demanded that a high-temperature durability of turbine stationary and moving blades exposed to a high-temperature combustion gas be improved. Required is a high-temperature durability, particularly the durability against the high-temperature corrosion induced by S in a fuel and Na, K or the like in the air for combustion. As a measure for preventing such a high-temperature corrosion, a method of coating alloy exhibiting an excellent high-temperature anticorrosive property is typically put into practice.
Further, as a matter of course, a metal temperature of the blade base material increases concomitantly with a rise in temperature of the combustion gas. There is, however, a limit in terms of a strength of a heat resistant material against the high temperature. Hence, a technology of cooling the blade remarkably advances. Consequently, the blade is constructed of a heat resistant alloy which assumes a hollowed structure and is small in wall thickness. The reduction in wall thickness of the blade because of the high-temperature corrosion remarkably spoils a high-temperature reliability of the blade.
Besides, a method of cooling the blade involves the use of a return flow, impingement, etc., thereby decreasing the metal temperature of the blade base material. However, the complicated cooling method is employed, and hence the uniform cooling over the blade becomes difficult. A distribution of temperatures is often produced.
Under such circumstances,a variety of anticorrosive coating materials and coating methods are proposed. The following is the method which has been used most frequently. Cr and Al are added to Co or Ni and an alloy of a combination thereof. Further, the blade is coated with an alloy to which Y and other rare earth elements are added (hereinafter referred to as an MCrAlX alloy. M implies Fe, Ni and Co, while X implies Y and other rare earth elements.) In the turbine blade coated with such an MCrAlX alloy, if under a high-temperature corrosion environment, the oxidation reaction of Cr, Al precedes the sulfidization reaction of Ni or Co, with the result that oxides of Cr, Al are produced. A sulfide of Ni or Co is a compound having a low melting point and easily assumes a liquid phase. Then, the reaction is promoted, and the wall is largely reduced.
On the other hand, the oxides of Cr, Al have a high melting point but do not assume the liquid phase. Therefore, the oxide is faster in formation reactive speed than the sulfide, and the degree of wall-reduction is reduced. Namely, MCrAlX alloy coating has greater Cr and Al contents than the heat resistant alloy. The oxidation of Cr, Al under the high-temperature corrosion environment is caused, and the high-temperature anticorrosive property is excellent with a less wall-reduction.
Further, as a result of this, the alloys containing much Cr, Al are required for MCrAlX alloy coating which exhibits more excellent high-temperature anticorrosive property. However, if the contents of Cr, Al increase for MCrAlX alloy coating, a toughness of alloy coating declines, thereby easily causing damages such as cracks or the like. If cracks are caused in the coated layer, the damage originating from the cracks advances to the blade base material, whereby the blade constructed thin is broken down.
In order to correspond to the deterioration of the high-temperature corrosion environment condition concomitant with the rise in the combustion gas temperature and the changes in the blade structure, a variety of improvements have been proposed as compared with the turbine blades having a low combustion gas temperature (in this case, no cooling is effected, or the cooling structure is simple, while the blade wall thickness is large). In techniques disclosed in, e.g., U.S. Pat. No. 4,080,486, U.S. Pat. No. 4,246,323 and U.S. Pat. No. 4,326,011, the contents of Al, Cr and Si in the vicinities of the surface portions of MCrAlX alloy coatings are increased. These methods depend chiefly on diffusive permeation. Proposed according to those methods is that the high-temperature anticorrosive property of MCrAlX alloy coating can be ameliorated by forming surface layers containing much Al, Cr and Si.
Further, the contents of Al, Cr, Si of the lower portion of the alloy coating are less than in the vicinity of the surface portion. There is no decline of toughness in the lower portion, and it is therefore predicted that if the cracks are produced in the surface portion, the advancement thereof stops at the lower portion.
However, any of those known improved techniques about the anticorrosive property of MCrAlZ alloy coating has attained reforming of only the surface portion of MCrAlX alloy coating of a single composition. As a result of examination by the present inventors, it have proven that those gas turbine blades are not necessarily sufficient for the combustion gas temperature.